1. Field of the Invention
The present invention relates to a process for producing a rubber powder from a vulcanized rubber material.
In particular, the present invention relates to a process for pulverizing a vulcanized rubber material by using a grinding device.
More in particular, the present invention relates to a process for pulverizing a vulcanized rubber material comprising a discarded rubber material.
Even more in particular, the present invention relates to a process for pulverizing a vulcanized rubber material comprising a discarded rubber material including discarded tyres previously torn to shreds.
2. Description of the Related Art
The increased production of industrial rubber products has resulted in the accumulation of large amounts of rubber wastes which per se do not find any practical applications and are generally disposed in dedicated landfills with the main drawbacks of environment pollution as well as of the need for large dedicated areas for storing said wastes.
Therefore, the reclaiming of vulcanized rubber material into a product, which can be advantageously reused, is a widely discussed issue and a long-felt problem to be solved.
Used vulcanized rubber material, such as waste rubber, old tyres and industrial rubber products, can be comminuted and added to rubber mixtures to be employed in a plurality of applications. This is particularly advantageous since important amounts of used vulcanized rubber material can be reused and, moreover, corresponding remarkable amounts of raw materials can be saved by replacing them with said discarded material.
The use in a rubber composition of a comminuted vulcanized rubber, whose particle size generally does not exceed 500 μm, does not remarkably impair the quality of the final product.
However, according to the known technologies available on the market, fine powders can be obtained from rubber material at the expense of large amounts of energy.
Reclaiming processes of used rubber material which are currently employed include: chemical reclaiming processes such as pyrolysis and devulcanization; thermal reclaiming processes such as extrusion, injection moulding and pressure moulding; mechanical reclaiming processes such as granulation, densification, agglomeration and pulverization.
Document U.S. Pat. No. 4,090,670 discloses the recovery of rubber from scrap vulcanized rubber tyres by devulcanization of the rubber tyres and subsequent removal of the devulcanized material, e.g. by rasping. The devulcanization is obtained by raising the surface temperature of the vulcanized rubber material.
Document U.S. Pat. No. 4,968,463 discloses the reclamation of thermoplastic material including the steps of: shredding to about one hundred millimiters, grinding to under about 40 millimeters, drying, pre-heating from 80° C. to 160° C., kneading at 120° C. to 250° C. and injection moulding or extruding.
A method of pulverizing natural or synthetic rubber materials is known, for instance, from document U.S. Pat. No. 3,190,565 which discloses the comminution thereof in mills provided with knife blades in the presence of antiagglomerating agents (in the form of polyolefin fines) that inhibit the sticking of the comminuted material to the cutting blades.
A further method of making powders from industrial rubbers consists in comminuting thereof by means of shear forces.
Document GB-1,424,768 discloses a plunger-type device provided with a rotating member so that the rubber material is crushed in the minimal gap between the rotating member and the inside wall of said device.
Document U.S. Pat. No. 4,157,790 discloses a process for the production of rubber powder having a particle size of from about 200 to 1,500 μm, said process comprising the step of providing small additions of powdering agents in order to obtain adequate fluidity of the rubber powder. A carrier gas is used during size-reduction which is carried out, for instance, by the grinding plates of a Pallmann mill. The increase in temperature which occurs in the size-reduction machine is minimized by cooling the carrier gas, e.g. at a temperature of about 5° C.
Document U.S. Pat. No. 4,650,126 discloses a process for grinding to a particle size of less than about 1 mm in diameter a soft and tacky polymeric material in the presence of a grinding aid in an attrition mill having counter-rotating grinding elements adjustably spaced apart. The mill temperature is adjusted so that nearly all of the grinding aid is retained on the softened polymer particles, thus improving polymer flow and reducing to a minimum the amount of loose grinding aid to be disposed of. Air is drawn through the mill to serve as the material carrier medium and at the same time to cool the mill, if required.
Document U.S. Pat. No. 2,412,586 relates to the fine grinding of rubber scrap with high grinding and screening efficiency. Said document discloses a cyclic process in which the rubber stock, with preliminary chopping if necessary, is continuously fed to a grinding mill together with a regulated amount of water and then subjected to the grinding operation in the presence of the added water, the resulting ground stock passed by a conveyer to a screen for screening out the fine particles, and the oversize material from the screen returned to the grinder for further grinding. According to said document the amount of water required for most efficient grinding or screening varies somewhat with the type of scrap being ground and with the fineness to which the scrap is to be ground.
A further method of producing finely dispersed powders from used rubber materials is the cryogenic destruction (e.g. Chemical Technology, Cryopulverizing, T. Nazy, R. Davis, 1976, 6, N° 3, pages 200-203). According to said method the rubber material is cooled to very low temperatures by using liquid nitrogen or solid carbon dioxide and then subjecting the cooled material to impact or cutting. This method produces finely dispersed powders having particle dimensions less than 500 μm, but it is very expensive due to the presence of a plant dedicated to liquid nitrogen production.
A further method of making powders from rubber materials consists in using an extrusion device of the single-screw or multiple-screw type.
For instance, document U.S. Pat. No. 4,607,797 discloses the pulverization of used polymers in an extrusion apparatus wherein the used material is heated to above its melting temperature in a first zone of said extrusion apparatus and cooled to below its solidification temperature with simultaneous pre-crushing and pulverizing of the solidified material in a second zone of said apparatus to form a powdered material. The action of the screw of the extruder is used to convey the material through the barrel of the extruder, while pulverizing disks mounted on the screw in the second cooling zone perform the pre-crushing and pulverizing of said material.
Document U.S. Pat. No. 5,743,471 discloses an extruder for solid state shear extrusion pulverization of polymeric materials comprising a feed zone, a heating zone adjacent to the feed zone, a powder formation zone adjacent to the heating zone and a powder discharge zone adjacent to the powder formation zone. Furthermore, the extruder is provided with temperature adjustment means for heating the polymeric material to a temperature lower than the decomposition temperature of the polymeric material in the heating zone and for maintaining the polymeric material below its melting point in the powder formation zone, but at a temperature above its glass transition temperature in the powder formation zone to inhibit the formation of agglomerates.
Documents U.S. Pat. Nos. 4,607,796; 5,395,055; 5,704,555 and JP 6-179215 disclose further processes according to which the extruder is provided with heating and cooling zones.